Thromboresistant coated medical device

ABSTRACT

Coatings are provided in which biopolymers may be covalently linked to a substrate. Such biopolymers include those that impart thromboresistance and/or biocompatibility to the substrate, which may be a medical device. Coatings disclosed herein include those that permit coating of a medical device in a single layer, including coatings that permit applying the single layer without a primer. Suitable biopolymers include heparin complexes, and linkage may be provided by a silane having isocyanate functionality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates to the field of medical devices and moreparticularly to the field of coatings for medical devices.

2. Description of Related Art

Arteriosclerosis is a condition that detrimentally affects manyindividuals. Untreated, arteriosclerosis may lead to severeconsequences, including heart damage, heart attack and death. Knowntreatments for arteriosclerosis have had limited success.

Transluminal balloon angioplasty, wherein a balloon is inserted via acatheter into the artery of the patient and expanded, therebysimultaneously expanding the partially closed artery to a more openstate, is a well-known treatment for arteriosclerosis, but long-termbenefits of balloon angioplasty are limited by the problems of occlusionand restenosis, which result in re-closure of the artery.

A variety of intravascular stents and prostheses have been developed tosupport diseased arteries and thereby inhibit arterial closure afterangioplasty. In particular, expandable intraluminal stents have beendeveloped in which a catheter is used to implant a stent into the arteryof the patient in a minimally invasive manner.

Like other foreign bodies placed into arteries, stents can result incoagulation or thrombosis in the intravascular environment. Thrombosiscan inhibit blood flow through the stent, diminishing its effectiveness,or can cause clotting, which can threaten the life of the patient.Accordingly, methods of reducing thrombotic activity have been sought toreduce the negative side effects caused by certain stents.

A number of coatings have been developed for medical devices that areintended to promote compatibility between a particular medical deviceand the environment in which the medical device resides. Some of thesecoatings, known as thromboresistant coatings, are intended to reduce thethrombosis often associated with insertion of a foreign object, such asa medical device, into the interior of the body.

Heparin, or heparinic acid, arteven, or leparan, is a glycosaminoglycanwith well-known anticoagulant activity. Heparin is biosynthesized andstored in mast cells of various animal tissues, particularly the liver,lung and gut. Heparin is known to have antithrombotic activity as aresult of its ability to bind and activate antithrombin III, a plasmaprotein which inhibits several enzymes in the coagulation cascade. Ithas been hoped that heparin coatings, by inhibiting thrombogenesis, canimprove the therapeutic outcomes derived from intra-vascular medicaldevices, such as stents.

However, known heparin coatings are subject to a number of defects,including incompatibility with the organism and/or microscopic featuresof the surface to be coated, excessive thickness, difficulty inapplication, and insufficient durability. For example, several knowncoatings are based upon simultaneous coulombic interactions betweenheparin and tri(dodecyl)methylammonium chloride, which is also referredto herein as heparin-TDMAC, and hydrophobic interactions between thequaternary ammonium ion of heparin-TDMAC and the surface of the device.Due to the relative weakness of hydrophobic interactions, such coatingstypically leach away from the substrate to which they are applied withina few hours; coatings of this type, therefore, are not generally durableenough to provide beneficial therapeutic results.

Other known coatings comprise silanes having a pendent amino or vinylfunctionality. In the fabrication of these coatings, a base layer ofsilane is applied initially to the surface, followed by the applicationto the base layer of a second layer comprising antithrombogenicbiomolecules, such as heparin. It is necessary that the pendent groupsof the base layer of silane be both complementary and accessible togroups on heparin. In some such coatings, a silane with terminal aminofunctionality is applied to a substrate to form a first layer, followedby application of heparin in solution to form the second layer. Incertain examples of this strategy, the amino functionality of the silanebase layer reacts with an aldehyde-containing heparin derivative to forma Schiff base and thereby covalently attach the biomolecule to the baselayer. In another group of coatings of this general class, a base layercomprising a silane with a vinyl functional group is applied to asurface, followed by covalent attachment, via free radical chemistry, ofa heparin-containing derivative to the base layer.

Some of the known coatings have been found lacking in bioeffectivenessand stability. Modifications made in these coatings utilize additionalcoatings of polymeric matrices comprising reactive functionalities. Themulti-step process required to fabricate the polymeric matricesnecessary in these approaches increases the thickness of the resultingcoatings. Thick coatings present a number of difficulties. First, thickcoatings increase the profile of the medical device in the intravascularenvironment. A stent with a thick profile, for example, can reduce bloodflow, thereby undermining the therapeutic benefit of the stent. A thickcoating may also render the coating itself more vulnerable to pitting,chipping, cracking, or peeling when the stent is flexed, crimped,expanded, or subjected to intravascular forces. Any of the foregoingresults of excessively thick coatings may reduce the antithrombogeniccharacteristics of the stent. Moreover, the likelihood of pitting ishypothesized to be greater in thick coatings, and pits in a coating mayincrease the susceptibility to galvanic corrosion of the underlyingsurface. Because their fabrication requires additional steps, coatingscomprising multiple layers may also be more difficult and expensive tomanufacture.

Accordingly, a need exists for a thromboresistant coating that is thin,durable, and biocompatible, and that may be applied in a single coating.

SUMMARY OF THE INVENTION

Coatings are provided herein in which biopolymers may be covalentlylinked to a substrate. Such biopolymers include those that impartthromboresistance and/or biocompatibility to the substrate, which may bea medical device. Coatings disclosed herein include those that permitcoating of a medical device in a single layer, including coatings thatpermit applying the single layer without a primer. It should beunderstood that it may be advantageous in some circumstances to applydouble layers of the coatings, such as to cover an area of a medicaldevice that is used to hold the device while a first layer is applied.Thus, single, double and multiple layers of coatings are encompassed bythe coatings disclosed herein.

The coatings disclosed herein include those that use an adduct ofheparin molecules to provide thromboresistance. The heparin moleculesmay comprise heparin-tri(dodecyl)methylammonium chloride complex. Usesof the term “heparin” herein should be understood to include heparin, aswell as any other heparin complex, includingheparin-tri(dodecyl)methylammonium chloride complex.

The coatings described herein further include those that use a silane tocovalently link a biopolymer to a substrate. The coatings include thosederived from silanes comprising isocyanate functionality.

The disclosed coatings include those that can be applied without a baseor primer layer.

Coatings are also included that provide a thin and durable coatingwherein the thickness of said coating can be controlled by applicationof single or multiple layers.

Coatings are provided wherein thromboresistance activity can be modifiedby choice of appropriate amounts of heparin-TDMAC complex and silane.

Thin, durable coatings are provided having controllable bioactivity.

Single or multi-layer coatings disclosed herein are designed to impartthromboresistance and/or biocompatibility to a medical device. In oneembodiment, the coating provides for covalent linking of heparin to thesurface of the medical device.

One coating formulation of the present invention initially consists ofheparin-TDMAC complex, organic solvent and silane. Wetting agents may beadded to this formulation. A silane is chosen that has an organic chainbetween isocyanate and silane functionalities. The isocyanatefunctionality reacts with an amino or hydroxyl group on the heparinmolecule. After the reaction, the formulation contains covalent adductsof heparin and silane, in addition to organic solvent and otheradditives. Unreacted silane or heparin-TDMAC complex may be present inthe formulation, depending on the relative amounts of the reagentsutilized.

Once the coating formulation is applied to a device, the silane endgroup of the adduct mentioned above adheres to the substrate surface,and a network, or film, containing heparin-TDMAC complexes is created onthe surface of a substrate. Heparin molecules in the heparin-TDMACcomplex are known to have anticoagulant properties. When exposed toblood, heparin molecules inactivate certain coagulation factors, thuspreventing thrombus formation.

The direct adherence of the silane end group to the substrate means thatthe coating may be applied to a wide range of medical device materialswithout the use of a base/primer layer. The covalent bond between thesurface and the silicon of the silane comprising the heparin-TDMACcomplex provides superior durability compared to known coatings.

The coating can be applied by dip coating, spray coating, painting orwiping. Dip coating is a preferred mode.

The coating can be thin and durable. The coating thickness can becontrolled in a number of ways, e.g., by the application of single ormultiple layers. Since the coating process described herein may be aone-step process, coating thickness is not increased as a result of theneed to apply multiple layers, as in certain known coating methods.

The bioeffectiveness of the coatings can be controlled by selectingappropriate amounts of reactants. In particular, the thromboresistanceactivity of the coating can be controlled by modifying the amount ofheparin-TDMAC complex in the coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Single or multi-layer coatings are provided herein that are designed toimpart thromboresistance and/or biocompatibility to a medical device. Inan embodiment of the invention, the coating provides for the covalentlinking of heparin molecules to a substrate.

A heparin molecule is understood to contain a specific art-recognizedpentasaccharide unit that displays antithrombogenic qualities. Covalentlinkage of a heparin molecule to a surface is understood to affect atleast one, but not all, of the hydroxyl and amino moieties comprised bythat molecule; the covalently linked heparin, therefore, presents athromboresistant surface to the environment surrounding the coatedsubstrate. Different methods and formulations for covalently linkingheparin to the surface may affect different sites on the heparinmolecules, so that different formulations will provide different levelsof anti-thrombogenicity.

One coating formulation of the present invention initially consists ofheparin-TDMAC complex, organic solvent and a silane. Other biopolymersmay be used in place of or in addition to heparin-TDMAC complex, andsuch biopolymers may be covalently linked to a substrate according tothe present invention. Such biopolymers may be those that providethromboresistance, or those that have other desired bioactivity.

The silane provided may have functionality capable of reacting with anucleophilic group, e.g., a hydroxyl or amino group. In particular, thesilane may comprise isocyanate, isothiocyanate, ester, anhydride, acylhalide, alkyl halide, epoxide, or aziridine functionality. In certainembodiments described herein, the silane comprises isocyanatefunctionality.

The silane comprising isocyanate functionality may be linked covalentlyto any biopolymer that provides anti-thrombogenicity. The selectedbiopolymer may be selected from a group of heparin complexes, includingheparin-tridodecylmethylammonium chloride, heparin-benzalkoniumchloride, heparin-steralkonium chloride,heparin-poly-N-vinyl-pyrrolidone, heparin-lecithin,heparin-didodecyldimethyl ammonium bromide, heparin-pyridinium chloride,and heparin-synthetic glycolipid complexes. The selected biopolymer mayalso be another biopolymer that has hydroxyl or amine functional groupsthat can react with the isocyanate functionality of the silane.

The selected biopolymer is preferably capable of dissolving in anorganic solvent, as opposed to biopolymers that dissolve only in water.Solubility in organic solvents confers a number of advantages, e.g.,elimination of water-mediated decomposition of the isocyanate-containingsilane. In one preferred embodiment, the selected biopolymer isheparin-tri(dodecyl)methylammonium chloride complex.

Wetting agents and other additives may be added to the coatingsdescribed herein, to improve the adherence to the substrate, to improvethe ease of adding the coatings to a substrate, or for other purposes. Avariety of organic solvents may be used, including tetrahydrofuran(THF). Additives may include surface active agents, such as Triton.

The selected silanes may have an organic chain between the isocyanatefunctionality, which covalently links to the heparin molecule, and anend group that is capable of linking to a substrate surface. The endgroup may link to pendant oxide groups on the substrate surface; in somecases, the pendant oxide groups may be obtained by oxidation of thesubstrate.

The bioactivity, including thromboresistance, of the disclosed coatingsmay be selectively modified by controlling the amounts ofheparin-tridodecylmethylammonium chloride complex, silane comprisingisocyanate functionality, and organic solvent, as well as otherconstituents, to provide the desired thromboresistance. In an embodimentof the coatings, the concentration of the silane in the formulation isbetween about one-half percent and about four percent. In an embodiment,the concentration of heparin-tridodecylmethylammonium chloride in theformulation is between about one-tenth percent and about four percent.One preferred coating is a solution with a formulation of silane ofabout five-tenths percent and a formulation of theheparin-tridodecylmethyl-ammonium chloride complex of about two-tenthspercent. In one such preferred solution, the organic solvent istetrahydrofuran.

Heparin molecules, including those in heparin-TDMAC complex are known tohave anticoagulant properties. When exposed to blood, structuralelements of heparin molecules inactivate certain coagulation factors,thus preventing thrombus formation.

The coatings described herein may be applied in a single layer. Thelayer can be formed by reacting silane having isocyanate functionalitywith a heparin in an organic solvent to form a silane-heparin complex,which can be applied directly to a substrate, such as a metal substrate,in a single-layer coating that can be applied without a primer. Thesingle layer can thus be made sufficiently thin to minimize the problemsof peeling, cracking, and other problems that characterize some thickercoatings that require multiple layers, primers, or polymeric matricesfor binding to the substrate. Thus, the layers may perform better underthe mechanical crimping or expansion of a medical device, such as astent, to which they are applied, and may perform better in theintravascular environment.

The silane end groups of the monomer that yield the coatings react withoxides or hydroxyl groups on the surface of stainless steel. Thestainless steel surface may be oxidized or cleaned and pre-treated, suchas with sodium hydroxide, to increase the number of appropriate sitesfor linking the silane end groups.

To improve hydrolytic stability, non-functional silanes can be added tothe formulations disclosed herein. Other silanes may be used to link tosubstrates, such as trihalosilanes, and silanes having methoxy andethoxy groups. Silanes having triethoxy, trialkoxy, trichloro, and othergroups may be provided to yield the covalent linkages present in thecoatings disclosed herein. The non-functional silanes may be selectedfrom the group consisting of chain alkyltrialkoxysilanes andphenyltrialkoxysilanes.

In an embodiment, the amount of functional silane is preferably selectedto provide substantially complete coverage of the substrate surface;that is, it may be desirable to have the single layer cover all of thesurface that would otherwise be exposed to the environment in which thesubstrate will be placed.

The adherence of the silane end group to the substrate means that thecoating may be applied to a wide range of medical device materialswithout the use of base/primer layer. The covalent bond between theheparin-TDMAC complex and the substrate provides a thin and durablecoating. The coating's thickness can be controlled, e.g. by choice ofthe length of the chain connecting the silane and isocyanatefunctionalities.

The bioeffectiveness and/or bioactivity of the thromboresistant coatingcan be controlled by selecting appropriate amounts of reactants. Inparticular, the thromboresistance activity of the coating can bemodified by modifying the amounts of heparin-TDMAC complex and silane inthe coating.

Single layers have further advantages in that problems may arise in theextra steps required for the deposition of multiple layers. For example,dust or other particulates may appear between coatings in two-stepprocesses. Also, application of a second layer may tend to reducereactivity of the first layer in an unpredictable way.

Coatings of the present invention may be applied to medical devices thatare placed in the body of a human, or that remain outside the body.Coated medical devices that are placed in the human body may includestents, cathethers, prostheses and other devices. Coated medical devicesthat remain outside the human body may include tubing for the transportof blood and vessels for the storage of blood. Substrates or medicaldevices on which the coatings described herein may be applied caninclude a wide variety of materials, including stainless steel, nitinol,tantalum, glass, ceramics, nickel, titanium, aluminum and othermaterials suitable for manufacture of a medical device.

The coatings disclosed herein may further include a film-forming agentfor the coating. The film-forming agents could slow any leaching of thebiopolymer from the coating. The film forming-agent could be added in asecond layer, or dissolved simultaneously with the silane and thebiopolymer. Appropriate film-forming agents could include celluloseesters, polydialkyl siloxanes, polyurethanes, acrylic polymers orelastomers, as well as biodegradable polymers such as polylactic acid(PLA), polyglycolic acid (PGA), copolymers of PLA and PGA, known asPLGA, poly(e-caprolactone), and the like.

To create coatings of the present invention, the silanes and heparincomplex are dissolved in a solvent, which may be an organic solvent. Thesolutions preferably should be substantially anhydrous, because watertends to react with isocyanate groups of the silane molecule. The watermay be added after mixing the silane-isocyanate with heparin. In certainembodiments, the silane and heparin are combined in solution, theresulting solution is aged for about one day, the pH is adjusted with aweak acid, and then water is added to hydrolyze silane. The pH of thesolution may be adjusted with aqueous acetic acid. Instead of addingwater, it is possible to hydrolyze the silane groups by exposure tomoist atmospheric conditions. It is desirable to mix the silane andheparin complex in a manner so as to include a slight excess of heparinmolecules, so that all of the isocyanate is reacted, preventing adversereactions between the isocyanate and any water. Moreover, it isdesirable to have a single heparin react with each silane isocyanatefunctional group; this goal is most easily accomplished by starting withan excess of heparin.

Based on experimental results, it was found that, in certainembodiments, solutions of about two-tenths percent heparin complex andabout five-tenths percent silane provided effective coatings. However,coatings in a fairly wide range may be effective. Thus, coatings arelikely to have some effectiveness in cases in which heparin complex ispresent in concentrations ranging from about one-tenth of a percent toabout twenty percent. Coatings with heparin in concentrations of lessthan ten percent may be preferable in some formulations. Coatings withheparin in concentrations of less than five percent may be preferable inother formulations. Coatings may be expected to be effective informulations in which silane is present in a wider range ofconcentrations as well, including concentrations ranging from aboutone-tenth of a percent silane to about twenty percent silane.

The thromboresistant characteristics of heparin coatings can be assessedqualitatively and quantitatively, so that methods can be developed thatprovide uniform coating with a desired amount of bioactivity.Successfully heparinized surfaces give a purple stain when exposed totoluidine blue. After coating, the surface is exposed to a salinesolution for a number of days or weeks, and thromboresistance activityis measured as a function of time. Stents and coupons coated asdisclosed herein were shown experimentally to display long-livedthromboresistant properties; bioactivity persisted for periods on theorder of months, and it will probably endure much longer.

The heparin activity of a sample may be quantified based on its abilityto inactivate thrombin. To quantify heparin activity in experimentalassays, heparin may be first mixed with human antithrombin III, whichbinds to create a complex. The heparin-antithrombin III complex can thenbe mixed with thrombin to produce a ternary complex comprising heparin,thrombin, and antithrombin. The heparin then departs this complex and isfree to react again with available antithrombin and thrombin to createadditional thrombin-antithrombin complexes. Thus, heparin acts as acatalyst for the antithrombin-mediated deactivation of thrombin. Thereaction of the active thrombin still left in the solution with asubstrate produces a proportional amount of p-nitro aniline exhibitingcolor. Thus, an assay may be conducted for a spectrophotometric analysisof color, to determine the amount of thrombin left in solution. The morethrombin left in solution, the lower the bioactivity of the heparin. Alow level of thrombin in solution indicates a high degree of catalysisof the thrombin-antithrombin reaction, which indicates a high level ofthromboresistance provided by the heparin. A baseline comparison for theassay is the very slow reaction of thrombin-antithrombin in the absenceof heparin. The results of the assay can be quantified usingspectrophotometry. The assay mimics the reactions that occur in thehuman bloodstream, where thrombin and anti-thrombin circulate at alltimes. The reaction between antithrombin and thrombin in the body, whichis catalyzed by the heparin of the coatings of the present invention,helps suppress the coagulation that results from thrombogenesis on amedical device.

Various methods of making coatings of the present invention arepossible, and examples of such methods and certain resulting coatingsare as follows. Such methods and coatings are disclosed by way ofexample, and are not intended to be limiting, as other examples may bereadily envisioned by one of ordinary skill in the art. The followingexamples include methods of providing coatings of the present inventionin a single layer, without the need for a primer layer, as well asmethods of controlling the bioactivity of the resulting coating. In someinstances, experimental results are provided showing sustainedbioactivity for the particular coating.

Coatings can be applied in a wide variety of conventional ways,including painting, spraying, dipping, vapor deposition, epitaxialgrowth and other methods known to those of ordinary skill in the art.

To test coatings disclosed herein, infrared scans were performed todemonstrate changes in the isocyanate functionality, through observationof the isocyanate peak (NCO, 2260 or 2270 cm-1) over time.Isocyanatosilane was formulated with different components, includingheparin-tridodecylmethylammonium chloride complex(Heparin-TDMACcomplex), tetrahydrofuran (“THF”) and Triton (an optional, surfaceactive agent) in solution to determine whether the intensity of theisocyanate peak changed over time. Table 1 shows the observation of theisocyanate functionality for different solution constitutents:

TABLE 1 Solution Observation 1) Silane + THF No change in peak with time2) Silane + THF + TDMAC No change in peak with time 3) Silane + THF +Triton No change in peak with time 4) Silane + THF + Heparin-TDMAC Peakdisappears with time complex depending on the concentration of silaneand heparin-TDMAC complex

The observation that the isocyanate peak disappears with time in thesolution that includes silane, THF and Heparin-TDMAC complex suggeststhat a reaction occurs between functional groups on heparin and theisocyanate group of silane.

In embodiments of the present invention, the coating formulationcontains the following constituents, which may vary in concentrations indifferent embodiments: Heparin-TDMAC complex, an organic solvent, suchas THF, a silane, such as 3-isocyanatopropyl triethoxysilane(OCN—(CH₂)₃-Si(OEt)₃), and Triton (x-100). In a first embodiment, asolution of these constituents was mixed and allowed to sit in order topermit a reaction to occur. Allowing the solution to sit for one dayallowed the reaction to occur, but shorter reaction times may well beeffective. Before coating the substrate with the solution, the pH wasadjusted. Solutions of the above constituents were adjusted to a pHbetween 4.5 and 5.5 using a solution of acetic acid and water. Afteradjusting pH, it is desirable to wait for a period of time, such asfifteen minutes, before applying the coating. Once the coating wasapplied, it was dried in air and cured in an oven. In particular,coatings of the above constituents were dried in air for about twentyminutes and then cured in an oven at eighty-five degrees Celsius forabout one hour.

Coatings, derived from the above-described solutions, on coupons andstents were tested in various ways. First, as a qualitative test, coatedcoupons and stents were dipped in toluidine blue solution and then werescreened for the presence of a purple stain. As mentioned above, thepresence of a purple stain in this assay indicates the presence ofheparin in the sample being assayed. Additionally, the intensity of thepurple stain observed in this assay is proportional to the amount ofheparin in the sample. Therefore, a comparison of the intensities of thepurple stains produced in this assay by a set of samples allows anassignment of the relative amounts of heparin comprised by the coatingsof those samples.

As a quantitative test for heparin activity, a heparin activity assaywas conducted according to a conventional thrombin inhibition assaytechnique. The heparin assay permitted determination of the ability ofthe heparin coating to deactivate thrombin and thus to providethromboresistance. The purpose of the protocol was to assay for heparinactivity based on thrombin inhibition. A number of different reactionsare understood to take place in order to determine heparin activity. Inthe first reaction:

Heparin+ATIII (excess)—[Heparin*ATIII]

Heparin reacts with Human Antithrombin III (“ATIII”) to yield aHeparin-Antithrombin III complex. In the second reaction:

[Heparin*ATIII]+Thrombin (excess)—[Heparin*ATIH*Thrombin]+Thrombin

the Heparin-Antithrombin complex reacts with Thrombin to yield aHeparin-Antithrombin-Thrombin complex. In the third reaction:

S2238+Thrombin—peptide+p-nitroaniline (measured at 405 nm)

the amount of the thrombin was measured. As a result, the size of thep-nitroaniline peak measured at 405 nm is inversely proportional to theamount of heparin present.

Exemplification

The invention now being generally described, it will be more readilyunderstood by reference to the following examples which are includedmerely for purposes of illustration of certain aspects and embodimentsof the present invention, and are not intended to limit the invention.

General Procedures

In the following examples, heparin activity on coated coupons or stentswas measured after exposing the coated object to a continuous flow ofsaline at thirty-seven degrees Celsius for a selected time period.Stainless steel coupons and stents were cleaned before coating. Thecoupons or stents were cleaned with several organic solvents, such ashexane and isopropanol, followed by rinsing with distilled water. Thecleaning procedure was carried out in an ultrasonic bath for fifteenminutes. After this procedure, the coupons or stents were placed insodium hydroxide solution (1.0 N) for fifteen minutes and then washedthoroughly with distilled water. Samples were air dried before coating.

It should be noted that thrombin inhibition assay techniques arenotoriously subject to significant sample error; accordingly, it is notunusual to obtain variable experimental results for a given sample. Theexamples below identify results for multiple samples under a variety ofconditions and thus indicate in the aggregate that the coatingsdescribed herein are likely to provide therapeutic levels ofthromboresistance. However, results from any single formulation werefound to vary somewhat depending on particular sample conditions. Incases where more than one set of data is provided for a given sample,the individual data sets reflect measurements taken at distinctpositions on that sample; the data sets in these cases, therefore, donot necessarily reflect a lack of precision in the measurements.

EXAMPLE 1

Stainless steel coupons were coated with a formulation of 1%heparin-TDMAC complex, 2% silane and 97% THF. The coupons were dippedonce in the formulation, with a dwell time of five seconds at a coatingspeed of 10 in/min, to give a single layer of coating. Results are setforth in Table 2.

TABLE 2 Activity, mU/cm² Sample Unwashed 7 days wash 97-080-90C <10 <1097-080-90C <15 <10 97-080-90D <15  <5 97-080-90D <15  <5

The coating showed toluidine blue stain before and after washing withwater. The coating showed heparin activity after one week of exposure tosaline.

EXAMPLE 2

Stainless steel coupons were dipped once, at coating speeds of 10 in/minand 42 in/min and for a dwell time of five seconds, and resulting insingle layer coatings of different thickness, in the followingformulations: 1) 7% heparin-TDMAC complex, 2% silane and 91% THF and asmall amount of Triton; and 2) 2% heparin-TDMAC complex, 2% silane and96% THF and a small amount of Triton. Sample pieces were cut fromcoupons and were either washed or not washed before being measured underthe indicated conditions after the indicated amounts of time. Resultsare set forth in Table 3:

TABLE 3 Activity, mU/cm² 1 day 2 days 1 day 2 days 7 days Sampleunwashed unwashed wash wash wash 97-100-9A <50 <75 <15 <25 <25 97-100-9A<50 <75 <15 <25 <25 97-100-9B <50 <75 <25 <50 <50 97-100-9B <75 <75 <15<50 <10

A toluidine blue stain was present before and after washing, and thecoupons showed heparin activity after seven days of washing. Combinedwith Example 1, the results showed that heparin activity can be variedusing different coating formulations and coating processes.

EXAMPLE 3

Stainless steel coupons were dipped once, at speeds of 10 in/min and 42in/min, and for dwell times of five seconds, two minutes and fifteenminutes, and resulting in coatings of different thickness, in thefollowing formulations: 1) 7% heparin-TDMAC complex, 2% silane and 91%THF and a small amount of Triton; and 2) 2% heparin-TDMAC complex, 2%silane and 96% THF and a small amount of Triton. Results are shown inTable 4.

TABLE 4 Activity, mU/cm² Sample 1 day unwashed 1 day wash 7 days wash97-100-15A <150  <10  <5 97-100-15A <100  <10 <10 97-100-15B  <50  <10<25 97-100-15B  <25  <1 <25 97-100-15C  <75  <25 97-100-15C <100  <5097-100-15D <150  <50 97-100-15D <150  <50 97-100-15E <150  <10 <1097-100-15E <150  <25 <25 97-100-15F <150  <10 <25 97-100-15F <200  <25<25 97-100-15G <150  <25 97-100-15G <150  <25 97-100-15H <150  <5097-100-15H <150  <50 97-100-15I <200 <100 <50 97-100-15I <200  <75 <7597-100-15J <200 <100 97-100-15J <250 <100

Seven day results were for certain pieces measured at the one day pointand then placed back into a flusher for additional days of washing.Toluidine blue stains were present before and after wash, with shadesdiffering with thickness. Heparin activity was present after seven daysof washing. In combination with Examples 1 and 2, this exampledemonstrated that heparin activity can be varied using different coatingformulation and coating processes.

EXAMPLE 4

Stainless steel coupons were dipped once, at speeds of 10 in/min fordwell times of one-half, one, two, five, ten and fifteen minutes, in thefollowing formulation: 2% heparin-TDMAC complex, 2% silane. 96% THF anda small amount of Triton. Certain coupons were dipped into toluidineblue solution and rubbed under water. The coupons were then redipped intoluidine blue and checked for the presence of a stain. Results areshown in Table 5.

TABLE 5 Toluidine blue stain before rub Toluidine blue stain afterSample Appearance test rub test 97-100-30A Good coating, thin Uniform,light Uniform, light 97-100-30B Good coating, thin Uniform, lightUniform, light 97-100-30C Good coating, thin Uniform, light Uniform,light 97-100-30D Good coating, thin Uniform, light Uniform, light97-100-30E Good coating, thin Uniform, light Uniform, light 97-100-30FGood coating, thin Dark gritty stain Uniform, light, some peeling

A qualitative assessment of the effect of different solvents on coatingwas also performed, by dipping a coated sample in solvent for 60 secondsand then washing it with water and staining it with toluidine blue.Results are shown in Table 6.

TABLE 6 Solvent Hot water (high Hot water (high Sample IPA Toluenepressure flow) pressure flow) Acetone 97-100-30G Good purple stain Nostain Light stain Light stain Good stain

Heparin activity is displayed in Table 7.

TABLE 7 Activity, mU/cm² Sample 1 day unwashed 1 days wash 97-100-30A<150 <25 97-100-30A <150 <25 97-100-30B  <75 — 97-100-30B  <75 —97-100-30C  <50 — 97-100-30C  <50 — 97-100-30D  <50 — 97-100-30D  <50 —97-100-30E  <10 — 97-100-30E  <25 — 97-100-30F  <10 — 97-100-30F  <25 —97-100-30G  <25 <25 97-100-30G  <25 <25

This example indicated that coating thickness may be dependent on dwelltime, that rubbing does not remove the coating as indicated by stainsafter rubbing, that washing with various solvents has a different effecton coating durability, and that heparin activity was present afterwashing. The example provided further evidence that heparin activity canbe varied using different coating processes.

EXAMPLE 5

Stainless steel coupons were dipped once, at speeds of 10 in/min, andfor dwell times of two and fifteen minutes, in the followingformulations: 1) 2% heparin-TDMAC complex, 4% silane and 94% THF and asmall amount of Triton; 2) 2% heparin-TDMAC complex, 8% silane and 90%THF and a small amount of Triton; 3) 4% heparin-TDMAC complex, 4% silaneand 92% THF and a small amount of Triton; and 4) Diluted 4%heparin-TDMAC complex, 4% silane and 92% THF and a small amount ofTriton.

Coated coupons were dipped in toluidine blue solution and rubbed withfingers under water, then redipped in toluidine blue and checked forstains. Results are displayed in Table

TABLE 8 Toluidine blue stain Toluidine blue stain after SampleAppearance before rub test rub test 97-100-36A (2 min) Good coatingUniform stain Uniform 97-100-36A (15 min) Good coating Uniform stainUniform 97-100-36B (2 min) Good coating Uniform stain Uniform 97-100-36B(15 min) Good coating Uniform stain Uniform 97-100-36C (15 min) Goodcoating Very thick, gritty Uniform, some peeling 97-100-36C (15 min)Good coating Very thick, gritty Uniform, some peeling 97-100-36D (2 min)Good coating Uniform stain Uniform 97-100-36D (15 min) Good coatingUniform stain Uniform, some peeling

Heparin activity for this example is displayed in Table 9.

TABLE 9 Activity, mU/cm² Coating (%/% 1 day 30 days 1 day 30 days 87days Sample heptdmac/silane) unwashed unwashed wash wash wash 97-100-36A(2 min dwell) 2.0/4.0 <150, <50 <25, <5 <1, <1 <125 <25 97-100-36B (2min) 4.0/8.0  <25, <25 <25 <25, <5 <1, <1 <25 97-100-36C (2 min) 4.0/4.0<175, <150  <50, <5 <1, <1 <150 <25 97-100-36C (2 min) Diluted, 4.0/4.0 <50, <100 <150  <25, <5   0, <1 <25

This example demonstrated that for thin coatings thickness is notstrongly dependent on dwell time. Also, rubbing does not remove thecoating, as indicated by stains after rubbing. Long term durability ofthe coating is evident from heparin activity results. Again, heparinactivity can be varied using different coating formulation andprocesses.

EXAMPLE 6

Stainless steel coupons were dipped once, at speeds of 10 in/min and fora dwell time of two minutes, in the following formulation: 2%heparin-TDMAC complex, 2% silane and 96% THF and a small amount ofTriton. The coupons were then either left unsterilized, or sterilizedwith ethylene oxide or gamma radiation,

Results for non-sterile coupons are in Table 10.

TABLE 10 Activity, mU/cm² Coating (%/% unwashed Unwashed 7 days 28 daysSample heptdmac/silane) Dip 7 days 28 days wash wash 97-100-66A 2.0/2.0Single <125, >10, >12 <10, <10 <2, <1 <100 97-100-66E 2.0/2.0 Single<100, >10, >16 <10, <10 <1, 0 <125

Results for ethylene oxide sterile coupons are in Table 11.

TABLE 11 Activity, mU/cm² Coating (%/% 1 day 14 days Sampleheptdmac/silane) Dip unwashed unwashed 1 day 14 days 97-100-66A 2.0/2.0Single >12 >16, >16 <15 <2, <2 97-100-66E 2.0/2.0 Single >12 >16, >16<10 <3, <2

Results for gamma radiation sterilized coupons are in Table 12.

TABLE 12 Activity, mU/cm² Coating (%/% 14 heptdmac/ 1 day 14 days 20days 1 day days 20 Sample silane) Dip unwashed unwashed unwashed washwash days 97-100-66A 2.0/2.0 Single <200, >16 >16 <20, <1, <1, <200 <20<1 <2 97-100-66E 2.0/2.0 Single <200, >16 >16    0, 0 <2, <2, <200 <2 <2

The resulting coatings were thin, with long term durability as evidentby heparin activity results. Sterilization did not appear to affectcoating properties, regardless of the sterilization mode.

EXAMPLE 7

Stainless steel coupons were dipped once, dipped twice, or dipped,washed, and then dipped again, at coating speeds of 10 in/min and fordwell times of two minutes, in the following formulations: 1) 0.5%heparin-TDMAC complex, 0.5% silane, 99% THF & small amount of Triton; 2)0.5% heparin-TDMAC complex, 2.0% silane, 97.5% THF & small amount ofTriton; 3) 2.0% heparin-TDMAC complex, 0.5% silane, 97.5% THF & smallamount of Triton; and 4) 2.0% heparin-TDMAC complex, 2.0% silane, 96%THF & small amount of Triton.

Heparin activity is shown in Table 13.

TABLE 13 Activity, mU/cm² Coating (%/% 12 days 18 days 12 day 18 day 26day 72 day Sample heptdmac/silane) Dip unwashed unwashed wash wash washwash 97-100-69A 0.5/0.5 Single >10 <175   0 <5 —   0 97-100-69B 0.5/0.5Double >10 <150 <2 <2 — <1 97-100-69C 0.5/0.5 Dip/wash/Dip >10 <125 <2<2 <1 <1 97-100-69D 0.5/2.0 Single <10  <75 <1 <5 — <1 97-100-69E0.5/2.0 Double  <5  <5 <1 <5 — <1 97-100-69F 0.5/2.0 Dip/wash/Dip  <2 <5 <2 <5 <2 <1 97-100-69G 2.0/0.5 Single —  <15 — <5 — <1, <197-100-69H 2.0/0.5 Double —  <5 — <5 — <1, <1 97-100-69I 2.0/0.5Dip/wash/Dip —  <2 — <5 <2, <2   0, <1 97-100-69J 2.0/2.0 Single — <150— <5 — <1, <1 97-100-69K 2.0/2.0 Double — <200 — <5 — <1, <1 97-100-69K2.0/2.0 Dip/wash/Dip — <250 — <5 <3, <2 <1, <1

The resulting thin coatings demonstrated heparin activity, includinglight stains before and after rubbing. The long term durability of thecoatings were evident through heparin activity results. Coatingproperties were variable according to different coating methods.

EXAMPLE 8

Stainless steel coupons were dipped twice, or were dipped, washed, anddipped again, at speeds of 10 in/min and for dwell times of two minutes,in the following formulations: 1) 0.5% heparin-TDMAC complex, 0.5%silane, 99% THF; and 2) 0.5% heparin-TDMAC complex, 2.0% silane, 97.5%THF. The pH of the coatings was adjusted using acetic acid.

Heparin activity is shown in Table 14.

TABLE 14 Activity, mU/cm² Coating (%/% 1 day Sample heptdmac/silane) Dipunwashed 1 day 43 days 97-100-93A 0.5/0.5 Double <75 <2 <2, <197-100-93B 0.5/0.5 Dip/wash/Dip <50 <3 <1, <1 97-100-93C 0.5/2.0 Double<50 <2 <2, <2 97-100-93D 0.5/2.0 Dip/wash/Dip  <1 <1 <2, <2

The resulting thin coatings demonstrated heparin activity, includinglight stains before and after rubbing. The long term durability of thecoatings was evident through heparin activity results. Coatingproperties were variable according to different coating methods.

EXAMPLE 9

Stainless steel coupons and stainless steel stents were dipped twice, orwere dipped, washed with saline and distilled water, and dipped again,at coating speeds of 10 in/min and for dwell times of two minutes.Coating pH was adjusted using hydrochloric acid. Coatings derived fromthe following formulations were prepared: 1) 0.5% heparin-TDMAC complex,0.5% silane, 99% THF; and 2) 0.5% heparin-TDMAC complex, 2.0% silane,97.5% THF.

Heparin activity is shown in Table 15.

TABLE 15 Activity, mU/cm² 11 43 Coating (%/% 1 day 11 days 1 day days 25days days Sample heptdmac/silane Dip unwashed unwashed washed wash washwash 97-100-92A 0.5/0.5 Double <25 <25, <25 <2 <1, <1 <1, <1, — <5, <2,<2, <2 97-100-92B 0.5/0.5 Dip/wash <25 <10, <25 <2 <1, <1 <2, <2, — /Dip<2, <2, <1, <2 97-100-92D 0.5/2.0 Double <10 <5 — <5, <2 <1, <297-100-92E 0.5/2.0 Dip/wash <25 <2 — <2, <2 <1, /Dip <1

Persistence of heparin activity after an increasing number of dayssuggests that most unattached heparin washes away immediately, but thatattached heparin does not easily wash away even after prolongedexposure.

Activity on stents is disclosed in Table 16.

TABLE 16 Activity, mU/cm² Coating (%/% 1 day Sample heptdmac/silane Dipunwashed 1 day 97-100-92C 0.5/0.5 Dip/wash/Dip <125 <50 97-100-92F0.5/2.0 Dip/wash/Dip  <50 <50

The resulting thin coatings showed light stains before and afterrubbing. The coatings were durable as evident from heparin activityresults. Coating properties were variable depending on different coatingmethods.

EXAMPLE 10

Stainless steel coupons and stainless steel stents were dipped, washedwith IPA and dipped again, at coating speeds of 10 in/min and for adwell time of two minutes, in the following formulations: 1) 0.1%heparin-TDMAC complex, 0.5% silane, 99.4% THF; and 2) 0.2% heparin-TDMACcomplex, 0.5% silane, 99.3% THF.

Heparin activity on coupons is shown in Table 17.

TABLE 17 Activity, mU/cm² Coating (%/% 2 days 2 days 34 days Sampleheptdmac/silane Dip unwashed wash wash 97-101-25A, Red 0.1/0.5 Double<25 <1 <2 97-101-25A, Red 0.1/0.5 Double <25 <1 <2 97-101-25B, green0.1/0.5 Dip/wash/dip <75   0 <2 97-101-25B, green 0.1/0.5 Dip/wash/dip<50   0 <2 97-101-25C, yellow 0.2/0.5 Double <50 <1 <5 97-101-25C,yellow 0.2/0.5 Double <25 <1 <5 97-101-25D, brown 0.2/0.5 Dip/wash/dip<50 <1 <2 97-101-25D, brown 0.2/0.5 Dip/wash/dip <25 <1 <2

Hepain activity on stents is shown in Table 18

TABLE 18 Activity, mU/cm² Coating (%/% 2 days 2 days 16 days Sampleheptdmac/silane Dip unwashed wash wash 97-101-25A, Red 0.1/0.5 Double<225  <5  <2 97-101-25A, Red 0.1/0.5 Double <225    0  <3 97-101-25B,green 0.1/0.5 Dip/wash/dip <125  <1  <2 97-101-25B, green 0.1/0.5Dip/wash/dip <100    0  <5 97-101-25C, yellow 0.2/0.5 Double <200 <15 <3 97-101-25C, yellow 0.2/0.5 Double <100  <5 <10 97-101-25D, brown0.2/0.5 Dip/wash/dip <200  <5 <10 97-101-25D, brown 0.2/0.5 Dip/wash/dip<225 <10  <5

The resulting thin coatings showed light stains before and afterrubbing. The coatings were durable as evident from heparin activityresults. Coating properties were variable depending on different coatingmethods.

EXAMPLE 11

Stainless steel stents were dipped once, at coating speeds of 10 in/minand for dwell times of five seconds and two minutes, in the followingformulations: 1) 4.0% heparin-TDMAC complex, 8.0% silane, 88% THF, smallamount of Triton; 2) 4.0% heparin-TDMAC complex, 4.0% silane, 92% THF,small amount of Triton; and 3) 2.0% heparin-TDMAC complex, 2.0% silane,96% THF, small amount of Triton.

Heparin activity is shown in Table 19.

TABLE 19 Coating (%/% Activity, mU/cm² Sample heptdmac/silane DipUnwashed 3/4 days 97-100-50A 4/8 Single <175  <50 97-101-50B 4/4 Single<150 <125 97-100-54B 2/2 Single <225  <25 (4 days)

Again, coating properties varied using different coating methods.

EXAMPLE 12

Stainless steel stents were dipped twice, at coating speeds of 10 in/minand at a dwell time of two minutes, in the following formulations: 1)0.2% heparin-TDMAC complex, 0.5% silane; 2) 0.5% heparin-TDMAC complex,0.5% silane; 3) 0.5% heparin-TDMAC complex, 1.0% silane; 4) 1.0%heparin-TDMAC complex, 1.0% silane; and 5) 1.0% heparin-TDMAC complex,2.0% silane. Stents were either left unsterilized or were sterilizedwith gamma radiation.

Table 20 shows results for non-sterile stents.

TABLE 20 Coating (%/% Activity, mU/cm² Sample # heptdmac/silane) Dip 4days unwashed 4 days 97-101-86A 0.2/0.5 Double <100 <1 97-101-86A0.2/0.5 Double <125 <1 97-101-86B 1.0/2.0 Double <200 <10  97-101-86B1.0/2.0 Double <225 <5 97-101-86C 1.0/1.0 Double <225 <5 97-101-86C1.0/1.0 Double <225 <5 97-101-86D 0.5/1.0 Double <200 <5 97-101-86D0.5/1.0 Double <225 <5 97-101-86E 0.5/0.5 Double <225 <5 97-101-86E0.5/0.5 Double <200 <5 97-101-86F 0.5/1.0 Sutton Double <125 <197-101-86F 0.5/1.0 Sutton Double <125 <5

Table 21 shows activity for sterile stents.

TABLE 21 Coating (%/% Activity, mU/cm² Sample # heptdmac/silane) Dip 4days unwashed 4 days 97-101-86A 0.2/0.5 Double >200 <1 97-101-86A0.2/0.5 Double >200 <5 97-101-86B 1.0/2.0 Double >200 <10  97-101-86B1.0/2.0 Double >200 <5 97-101-86C 1.0/1.0 Double >200 <10  97-101-86C1.0/1.0 Double >200 <10  97-101-86D 0.5/1.0 Double >200 <5 97-101-86D0.5/1.0 Double >200 <5 97-101-86E 0.5/0.5 Double >200 <5 97-101-86E0.5/0.5 Double >200 <5 97-101-86F 0.5/1.0 Sutton Double >200 <597-101-86F 0.5/1.0 Sutton Double >200 <5

Sterilization showed no effect on coating properties. The coatings weredurable on stents, as evident by heparin activity after several days ofwashing.

EXAMPLE 13

Several coupons and stents were coated with 0.2% heparin-TDMAC complex,0.5% silane and 99.3% THF. These pieces were sterilized by gammaradiation and sent to NAMSA for biocompatibility testing. Three tests,Hemolysis, Cytotoxicity and Thromboresistance, were conducted. Thecoating passed all three tests.

In addition to the foregoing examples, various other methods andcoatings may be envisioned in the spirit of the present disclosure. Forexample, heparin might be covalently linked to a substrate with a silaneidentified as capable of being soaked into a stainless steel surface.The silane compound could have amino or epoxy terminal groups. Thesilane could thus be used to link heparin molecules to the substrate ina manner similar to the silane of isocyanate functionality disclosedherein. Heparin could then be prepared with an aldehyde positive groupthat mixed with an NH2 group to provide an end linkable to heparinwithout affecting its activity. The procedure to make degraded heparinis well known to those of ordinary skill in the art.

A coating system may also be provided in which heparin can be covalentlylinked or can be incorporated into a matrix to obtain variable rate ofelution. A silicon fluid, such as Dow Corning MDX 4-4159 is used, withthe active silicon being an amino functional polydimethyl siloxanecopolymer. The coating may be used to coat stainless steel guide wires.This working can be utilized for heparin covalent-bonding as describedbelow.

First, a solution of heparin (deaminated) in water or other solvent maybe provided. A wire coated with a silicon fluid in a solvent may beplaced in the solution for some time, for example two hours. The heparinhas an aldehyde group that can link to the amino functionality in thesilicon copolymer. Other amino functionalized silicon polymers, orcopolymers, can be used to achieve covalent bonding of heparin to thesubstrate.

Equivalents

While the invention has been disclosed in connection with the preferredembodiments shown and described in detail, various modifications andimprovements thereon will become readily apparent to those skilled inthe art. Accordingly, the spirit and scope of the present invention isto be limited only by the following claims.

We claim:
 1. A medical device having a coating comprising the product ofthe reaction of: a silane having at least one functional group selectedfrom the group consisting of an isocyanate, an isothiocvanate, an ester,an anhydride, an acyl halide, an alkyl halide, an epoxide and anaziridine; and a biopolymer.
 2. The medical device of claim 1, whereinthe weight ratio of said silane to said biopolymer is from about 1:4 toabout 2:1.
 3. The medical device of claim 2, wherein said weight ratiois 1:4, 1:1 or 2:1.
 4. The medical device of claim 2, wherein saidbiopolymer is heparin or a complex thereof.
 5. The medical device ofclaim 4, wherein said biopolymer is selected from the group consistingof heparin-tridodecylmethylammonium chloride, heparin-benzalkoniumchloride, heparin stearalkonium chloride,heparin-poly-N-vinyl-pyrrolidone, heparin lecithin,heparin-didodecyldimethyl ammonium bromide, heparin-pyridinium chlorideand heparin-synthetic glycolipid.
 6. The medical device of claim 2,further comprising at least one additive selected from the groupconsisting of wetting agents, surface active agents and film formingagents.
 7. The medical device of claim 6, wherein said film-formingagent is selected from the group consisting of cellulose esters,polydialkyl siloxanes, polyurethanes, acrylic polymers, elastomers,biodegradable polymers, polylactic acid, polyglycolic acid, copolymersof polylactic acids, copolymers of polyglycolic acid andpoly(e-caprolactone).
 8. The medical device of claim 1, wherein saiddevice is selected from the group consisting of stents, catheters,prostheses, tubing and blood storage vessels.
 9. The medical device ofclaim 8, wherein said device is made of at least one material selectedfrom stainless steel, nitinol, tantalum, glass, ceramic, nickel,titanium or aluminum.
 10. The medical device according to claim 1,wherein said at least one functional group is an isocyanate.
 11. Themedical device according to claim 10, wherein said biopolymer is heparinor a complex thereof.
 12. The medical device according to claim 11,wherein said biopolymer is selected from the group consisting ofheparin-tridodecylmethylammonium chloride, heparin-benzalkoniumchloride, heparin stearalkonium chloride,heparin-poly-N-vinyl-pyrrolidone, heparin lecithin,heparin-didodecyldimethyl ammonium bromide, heparin-pyridinium chlorideand heparin-synthetic glycolipid.
 13. The medical device according toclaim 12, wherein said biopolymer is heparin-tridodecylmethylammoniumchloride.